1. Field of the Invention
This invention relates to a device for forming functional films, particularly semiconductive deposited films useful for electronic devices such as semiconductor devices, photosensitive devices for electrophotography, light input sensor devices for optical image inputting devices, etc.
2. Related Background Art
In the prior art, for formation of functional films, especially amorphous or polycrystalline semiconductor films, suitable film forming methods have been employed individually from the standpoint of desired material characteristics, uses, etc.
For example, for formation of silicon type deposited films such as of so called non-single crystalline silicon including amorphous and polycrystalline silicon which are optionally compensated for lone pair electrons with a compensating agent such as hydrogen atoms (H) or halogen atoms (X), etc., (hereinafter abbreviated as "NON-Si (H,X)", particularly "A-Si (H,X)" when indicating an amorphous silicon and "poly-Si (H,X)" when indicating a polycrystalline silicon) (the so called microcrystalline silicon is included within the category of A-Si (H,X) as a matter of course), there have been employed as attempts the vacuum vapor deposition method, the plasma CVD method, the thermal CVD method, the reactive sputtering method, the ion plating method, the optical CVD method, etc.
Particularly, as the method for forming a film of a photosensitive device for electrophotography, the plasma CVD (Chemical Vapor Deposition) method has been practicably applied.
This method comprises reducing a reaction chamber to high vacuum, feeding starting gases into the reaction chamber and thereafter decomposing the starting gases by glow discharging to thereby form a thin film on a substrate placed in the reaction chamber.
The amorphous silicon (A-Si) film formed by use of a silane gas such as SiH.sub.4, Si.sub.2 H.sub.6, etc. as a starting gas according to this method is relatively fewer in the localized level existing in the forbidden band of amorphous silicon (A-Si), and therefore not only valence electron control is possible by doping of a substitution type impurity but also it has excellent characteristics as a film of an electrophotographic photosensitive member.
FIG. 1 shows the basic constitution of the main portion of a preferred embodiment of a bulk production type vacuum film forming device for the plasma CVD method disclosed in the Patent Application already filed by the same applicant (U.S. patent application Ser. No. 491,799). Film formation on a cylindrical substrate surface according to this device is described in detail below.
111 is an intake chamber (substrate setting stage) for setting a cylindrical substrate 141 at a predetermined position, and by opening the door 115, one or a plural number of cylindrical substrates 141 are fixed on a fixing member 116.
The door 115 is closed, and the intake chamber 111 is internally reduced in pressure to a desired pressure by an evacuation system 131, and the cylindrical substrates 141 are heated by a heater 124 for heating the substrate to, for example, about 200.degree. to 300 .degree. C. After the temperature is sufficiently stabilized, the cylindrical substrates 141 are moved by a conveying means 117 into a relay chamber (relay stage) 112 maintained at a desired vacuum pressure by an evacuation system 142 by opening the intermediate gate valve 119. After movement, the gate valve is closed, and the gate valves 129 provided in the same number as the cylindrical substrates 141 are opened, and the cylindrical substrates 141 are descended by a vertically moving means 118, and the respective cylindrical substrates 141 are moved into a plural number of reaction furnaces (film forming stage) 141-1, 141-2 provided corresponding to the respective gate valves.
After the respective cylindrical substrates 141 are fixed on the respective receiving members 127 for cylindrical substrates rotatable by means of driving sources 137, the vertical moving means 118 is returned to the original position.
After the respective gates 129 are closed, the inner pressures of the reaction furnaces 114-1 and 114-2 are controlled appropriately as desired by means of an evacuation system 132 for the reaction furnaces 114-1 and 114-2 and an introduction system 134 of starting gases for formation of films such as silane, etc., and then high frequency voltage is applied on the substrates and coaxial cylindrical electrodes 126 by a high frequency power source 133 to generate plasma by discharging in the reaction furnaces 114-1 and 114-2 and decompose the starting gas such as silane introduced by the introduction system 134, thereby forming amorphous silicon films, etc., on the surfaces of the cylindrical substrates 141. During this operation, the cylindrical substrates 141 are heated internally by heaters 128, and rotated by the driving sources 137 to effect uniformization of film thickness. The plasma generated by discharging is confined by electrical shields 125 in the predetermined spaces in the reaction furnaces 114-1 and 114-2.
After completion of the film formation step, introduction of starting gases is stopped simultaneously with turning off the high frequency power source, and then, with the gate valves 129 being held up, the plural number of respective cylindrical substrates 141 having films formed thereon are drawn up by means of the vertical moving means 118 into the relay chamber 112, and thereafter the gate valves 129 are closed. Next, the gate valve 120 is opened, and the respective cylindrical substrates 141 having films formed thereon are moved by means of a conveying means 121 to a take-out chamber (substrate removing stage) 113 previously reduced in pressure to a predetermined pressure. After completion of movement, the gate valve 120 is closed again. The cylindrical substrate 141 moved to the take-out chamber 113 is cooled to a predetermined temperature under a predetermined reduced pressure by cooling action of the cooling plate 123 cooled by a cooling means 136. Then, the leak valve 139 is opened gradually so that no bad influence may be exerted on the film formed, and the take-out chamber 113 is communicated internally to the outer atmosphere and thereafter the take-out door 122 is upheld to take out the cylindrical substrates 141 having films formed thereon to the outside.
By repeating the film forming actuation steps as described above, film formation on a large number of substrates has been continuously practiced.
As described above, according to the plasma CVD method of the prior art, it has been necessary to introduce high frequency power onto cylindrical substrates and coaxial cylindrical electrodes in the reaction chamber in order to make electrical properties of deposited films and film thicknesses uniform.
For this reason, it has not been easy to form deposited films on one or more cylindrical substrates at the same time in the reaction chamber, and therefore the prior art technique was not without problem when productivity was required to be dramatically improved.
Also, when cylindrical substrates are used, as a measure to carry out film formation stably and with good efficiency according to the plasma CVD method, electrodes arranged concentrically relative to the cylindrical substrates are used in the reaction chamber. In this case, the deposited films are deposited on both of the cylindrical substrates and the coaxial cylindrical electrodes to film thicknesses of the same extent, whereby only a part of the starting gases could be deposited on the desired cylindrical substrates. Accordingly, the utilization efficiency of the starting gases was low to involve the problem that the cost of the deposited film was high.
Further, according to the plasma CVD method, film formation is effected by decomposition of the starting gases by the high frequency energy introduced externally, and therefore the high frequency can not be easily introduced into the reaction chamber with good efficiency, thus posing a problem that the production cost cannot be made further lower with ease.
Further, some problems have existed in operation of the steps, and also in installation investment. As to operation of the steps, for example, many parameters involved pose a problem for the following reasons. That is, referring to film forming parameters such as mutual relationships between substrate temperature, flow rates and flow rate ratios of the gases introduced, pressure during film formation, high frequency power, electrode structure, structure of reaction vessel, evacuation speed, plasma generating system, etc., there already exist a large number of parameters. In addition, other parameters also exist. Accordingly, for obtaining a deposited film of a desired quality, it is required to select the parameters and set their values strictly. And, because of the parameters strictly selected, if one of them, particularly the parameter concerned with formation of plasma is changed, namely the plasma state becomes unstable, the film formed will be badly influenced markedly, resulting sometimes in difficult utilization of the film depending on the use. And, as to the device design, since strict selection of the parameters is required as mentioned above, the structure becomes complicated of itself, and the design must be made so as to correspond to the parameters individually strictly selected according to the change in the scale, the kind of the device. For such reasons, the plasma CVD method, although it has been accepted presently the best method, involves the problems such that enormous installation investment is required for the device when stable bulk production of A-Si films having desired characteristics is desired to be performed and, in addition, that the management items of the steps are so many and complicated as described above and consequently it is difficult to lower the cost.
On the other hand, for example, photosensitive devices for electrophotography have become diversified and it is socially demanded that photosensitive devices for electrophotography constituted of stable A-Si films corresponding to the objects, uses to be applied while satisfying wholly the requirements such as various characteristics, etc. should be supplied steadily at low cost. There is the situation where development of the method and the device satisfying this demand is earnestly desired.
These are also applicable to film formation of other layers such as NON-Si (H,X) films containing at least one kind selected from among oxygen atoms, carbon atoms and nitrogen atoms.
Further, in the case of preparing a semiconductor device with a multi-layer structure, the interfaces formed between the respective layers may sometimes become the factor which worsens the characteristics of the semiconductor device obtained. For example, to refer to a photosensitive member for electrophotography as an example, it has a multi-layer structure having a long wavelength light absorbing layer (first layer, amorphous silicon germanium layer controlled in band gap to narrower by containment of Ge) 201, a charge injection preventive layer (second layer, amorphous silicon layer doped with B) 202, a photosensitive layer (third layer, amorphous silicon layer not doped) 203 and a surface protective layer (fourth layer, amorphous silicon carbide layer) 204 laminated on a substrate 200 made of aluminum, as shown in FIG. 2. Since the kinds, the flow rates of the starting gases or the discharging intensities of plasma for formation of the respective layers are extremely different, efforts have been made to reduce the influence of the interfaces formed between the respective layers generally by exchanging completely gases by stopping discharging between the first layer and the second layer, between the second layer and the third layer and between the third layer and the fourth layer, or by providing a varied layer formed by varying the gas species, flow rates or discharging intensities of plasma continuously and gradually. However, in any case, it has been demanded to change and improve interface characteristics further satisfactorily.
As described above, in formation of silicon type deposited films, the points to be solved still remain and it has been desired earnestly to develop a device for forming deposited film which is capable of bulk production by effecting energy-saving with a further lowered cost while maintaining its practicably utilizable characteristics and uniformity, and can also improve further the interface state of a deposited film with a multi-layer structure for photosensitive devices for electrophotography, etc.